1. Field of the Invention
The present invention relates to a switch-mode power supply (SMPS). In particular, the present invention relates to a switch-mode power supply for preventing current overflowing between the MOS transistors during switching.
2. Description of Related Art
In general, a switch-mode power supply (SMPS) converts a direct current (DC) supply voltage into one or more DC output voltages that have a greater or lesser magnitude than the DC supply voltage. SMPSs are widely used in power electronic devices, particularly battery-powered devices such as portable cellular phones and laptop computers. Typically, SMPSs have a normal operation mode and a standby operation mode. Power electronic devices consume a large amount of power in the normal operation mode, as compared to the standby operation mode. Power electronic devices may automatically enter the standby operation mode when a user does not use the device for a predetermined time, and may automatically enter the normal operation mode when the user resumes the use of the device.
FIG. 1 shows a conventional switching mode power supply. A control signal from a signal input SCL1 is transmitted to driving devices 10 and 20. Each of the driving devices 10 and 20 outputs a signal to drive a P-channel metal oxide semiconductor (PMOS) transistor M1 and a N-channel metal oxide semiconductor (NMOS) transistor M2 respectively. A source of the NMOS transistor M2 is connected to a main power source V1, and a source of the PMOS transistor M1 is connected to a standby power source V2. Each drain of the PMOS transistor M1 and the NMOS transistor M2 acts as a voltage output Vo.
When the control signal from the signal input SCL1 is in a high level, each of the driving devices 10 and 20 outputs a high level signal, so the PMOS transistor M1 is turned off while the NMOS transistor M2 is turned on, and the drain of the NMOS transistor M2 outputs a main voltage from the main power source V1. When the control signal from the signal input SCL1 is in a low level, each of the driving devices 10 and 20 outputs a low level signal, so the PMOS transistor M1 is turned on while the NMOS transistor M2 is turned off, and the drain of the PMOS transistor M1 outputs a standby voltage from the standby power source V2.
An ideal operation state of the conventional switching mode power supply is the PMOS transistor M1 and the NMOS transistor M2 being switched on or off in turn. However, parameters of the PMOS transistor and the NMOS transistor are different because of the manufacturing technology, the scheduling error may occur between the switching of the PMOS transistor and the NMOS transistor. As a result, there can be an overlap time during the switching time, that is, the PMOS transistor M1 and the NMOS transistor M2 may both be switched on at the same time, which causes a large current from the main power source V1 to the standby power source V2 that may ruin the PMOS transistor and the NMOS transistor.
What is needed is a switch-mode power supply for preventing current overflowing between the MOS transistors during switching.